As energy prices increase due to the depletion of fossil fuels and interest in environmental pollution escalates, the demand for environmentally-friendly alternative energy sources is bound to increase in the future. In particular, the demand for small-sized, high-capacity secondary batteries is increasing in response to trends toward multifunction, high performance, and miniaturization of mobile devices.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries, which are thin enough to be applied to products such as mobile phones, is very high. In terms of the material for batteries, on the other hand, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, which exhibit high energy density, discharge voltage, and output stability, is very high.
In addition, secondary batteries may be classified based on the structure of an electrode assembly configured to have a structure in which a positive electrode, a separator, and a negative electrode are sequentially arranged. Typically, the electrode assembly may be configured to have a jelly-roll (wound) type structure in which a long sheet type positive electrode and a long sheet type negative electrode are wound in the state in which a separator is disposed between the positive electrode and the negative electrode or a stacked type structure in which pluralities of positive electrodes and negative electrodes each having a predetermined size are sequentially stacked in the state in which separators are disposed respectively between the positive electrodes and the negative electrodes.
However, the above-mentioned conventional electrode assemblies have the following problems.
First, the jelly-roll type electrode assembly is prepared by winding a long sheet type positive electrode and a long sheet type negative electrode in a dense state such that the jelly-roll type electrode assembly has a circular or oval structure in section. As a result, stress, caused by expansion and contraction of the electrodes during charging and discharging of a battery, may accumulate in the electrode assembly and, when the accumulated level of stress exceeds a specific limit, the electrode assembly may be deformed. The deformation of the electrode assembly results in non-uniformity of a gap between the electrodes. As a result, the performance of the battery may be abruptly deteriorated and the safety of the battery may not be secured due to the generation of a short circuit in the battery. In addition, it is difficult to rapidly wind the long sheet type positive electrode and the long sheet type negative electrode while maintaining a uniform gap between the positive electrode and negative electrode, with the result that productivity is lowered.
Secondly, the stacked type electrode assembly is prepared by sequentially stacking a plurality of unit positive electrodes and a plurality of unit negative electrodes. For this reason, it is necessary to additionally perform a process for transferring electrode plates which are used to prepare the unit positive electrodes and the unit negative electrodes. In addition, large amounts of time and effort are required to perform the sequential stacking process, with the result that productivity is lowered.
In order to solve the above-mentioned problems, there has been developed a stacked/folded type electrode assembly having an improved structure, which is a combination of the jelly-roll type electrode assembly and the stacked type electrode assembly. The stacked/folded type electrode assembly is configured to have a structure in which pluralities of positive electrodes and negative electrodes having a predetermined size are stacked in the state in which separators are disposed respectively between the positive electrodes and the negative electrodes so as to constitute a bi-cell or a full-cell and then a plurality of bi-cells or a plurality of full-cells is folded using a long separator sheet. The details of the stacked/folded type electrode assembly are disclosed in Korean Patent Application Publication No. 2001-0082058, No. 2001-0082059, and No. 2001-0082060, which have been filed in the name of the applicant of the present patent application.
However, the stacked/folded type electrode assembly is manufactured by winding a plurality of bi-cells or full cells in one direction. As the number of bi-cells or full cells that are stacked is increased, the number of times that the separator sheet is overlapped is increased, with the result that the overall width of the electrode assembly is increased. In addition, as the number of times that the bi-cells or full cells are wound is increased, the likelihood of snaking is increased.
Therefore, there is a high necessity for technology that is capable of minimizing the increase in overall width of the electrode assembly, even when the number of bi-cells or full cells that are stacked is increased, and of preventing the occurrence of snaking at the time of winding the bi-cells or the full cells.